Doctoral Degrees (Institute for Wine Biotechnology)

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Browsing Doctoral Degrees (Institute for Wine Biotechnology) by Author "Du Plessis, Heinrich Wilbur"

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    Effect of non-Saccharomyces yeasts and lactic acid bacteria interactions on wine flavour.
    (Stellenbosch : Stellenbosch University, 2018-03) Du Plessis, Heinrich Wilbur; Jolly, Neil; Du Toit, Maret; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Wine aroma and flavour are important indicators of quality and are primarily determined by the secondary metabolites of the grape, by the yeast that conducts the primary fermentation and also the lactic acid bacteria (LAB) that performs malolactic fermentation (MLF). This is a complex environment and each microorganism affects the other during the wine production process. Therefore, the overall aim of this study was to investigate the interactions between Saccharomyces, non-Saccharomyces yeasts and LAB, and the effect these interactions had on MLF and wine flavour. Contour-clamped homogeneous electric field gel electrophoreses (CHEF) and matrix-assisted laser desorption ionization using time-of flight mass spectrometry (MALDI-TOF MS) were useful tools for identifying and typing of Hanseniaspora uvarum, Lachancea thermotolerans, Candida zemplinina (synonym: Starmerella bacillaris) and Torulaspora delbrueckii strains. Hanseniaspora uvarum strains had β-glucosidase activity and Metschnikowia pulcherrima strains had β-glucosidase and protease activity. Only Schizosaccharomyces pombe and C. zemplinina strains showed mentionable malic acid degradation. Candida stellata, C. zemplinina, H. uvarum, M. pulcherrima and Sc. pombe strains were slow to medium fermenters, whereas L. thermotolerans and T. delbrueckii strains were found to be medium to strong fermenters, comparable to S. cerevisiae. The effect of non-Saccharomyces yeast species on MLF varied and inhibition was found to be strain dependent. In a Shiraz winemaking trial where seven non-Saccharomyces strains were evaluated in combination with S. cerevisiae and three MLF strategies, the C. zemplinina and the one L. thermotolerans isolate slightly inhibited LAB growth in wines where yeast and LAB were inoculated simultaneously. However, the same effect was not observed during sequential inoculation of LAB. Mixed culture fermentations using non-Saccharomyces yeasts contained lower alcohol levels, and were more conducive to MLF than wines produced with S. cerevisiae only. Yeast treatment and MLF strategy resulted in wines with significantly different flavour and sensory profiles. Yeast selection and MLF strategy had a significant effect on berry aroma, but MLF strategy also had a significant effect on acid balance and astringency of wines. In a follow up trial, H. uvarum was used in combination with two S. cerevisiae strains, two LAB (Lactobacillus plantarum and Oenococcus oeni) species and three MLF strategies. One of the S. cerevisiae strains had an inhibitory effect on LAB growth, while H. uvarum in combination with this S. cerevisiae strain had a stimulatory effect on MLF. Simultaneous MLF completed faster than sequential MLF and wines differed with regard to their chemical and sensory characteristics. Isoamyl acetate, ethyl hexanoate, ethyl octanoate, ethyl-3-hydroxybutanoate, ethyl phenylacetate, 2-phenyl acetate, isobutanol, 3-methyl-1-pentanol, hexanoic acid and octanoic acid were important compounds in discriminating between the different wines. Yeast treatment had a significant effect on fresh vegetative and spicy aroma, as well as body and astringency of the wines. The LAB strain and MLF strategy had a significant effect on berry, fruity, sweet associated and spicy aroma, as well as acidity and body of the wines. Mid-infrared (MIR) spectroscopy was used to differentiate between wines produced with the selected Saccharomyces and non-Saccharomyces yeast combinations, LAB species and MLF strategies. This study provides valuable information about the interactions between non-Saccharomyces, Saccharomyces yeast, LAB and MLF strategies, and how important pairing of strains are to ensure successful AF and MLF. Furthermore, the results also showed how these interactions can be applied to diversify wine flavour.